Whale vocalisation and ecolocation

Whales are the largest of all marine mammals. Their length varies from 3.5 to 27 meters and they can weigh up to 120 tonnes depending on the species. These huge animals usually live in the open oceans and feed on plankton, fish, squids or even other whales which can be found at depths of 400 m. As you go deeper in the sea the darker it gets, due to attenuation of light, therefore whales had to find a way, other than sight, to sense objects and to communicate with each other.

Sperm whale eating a giant squid

Communication is defined as the shearing of any kind of information between two or more individuals, whether it is beneficial or not. Due to the need to transmit information in water, the auditory and vocal systems of whales have developed by evolution and adaptation. Evidence can be found in the morphology and physiology of whales, for example, whales have two or three times the amount of fibres in the auditory nerve than in their optic nerve.

The evolutionary way that whales use to communicating amongs them, is achieved through vocalisation; this is when air passes from the lungs through specialised sound producing organs. These vocals, or “calls” as they are known, vary in sequence of sounds produced, frequency, duration and even silent intervals. These variations in calls may inform the receiver about the emotional state, the alertness level of the producer and many more factors. The calls will also vary due to the age and sex of the animal, the depth that they are produced, because of the different pressure, and their identity.

All the different calls of a whales, make up a “repertoire” of vocalisations. The repertoire is known to pass from the parents to the offspring at an early stage of life, but it’s not until puberty that they learn it all and can “sing” all the songs correctly. It has been observed that different pods of Orca whales, that are sympatric (they live in the same area), have different stereotypical calls that are unique for each pod. These stereotype calls are believed to be some kind of dialects. Although repertoires and calls are not understood by us, scientists think that there may be a number of calls that have the same meaning, and that a single call might actually have several meanings.

But how are these vocals produced? Although it is difficult to capture and keep a whale in captivity, one specific species has been studied in detail. This is the sperm whale (Physeter macrocephalus). Sperm whales have a large waxy organ called spermaceti that is situated anterior and dorsal of the skull. Bellow the spermaceti, the “junk” is found which consists of a series of fatty structures separated by columns of connective tissue. Scientists argue that it is the spermaceti that generates the equally spaced inter-pulse intervals. This is because as sound energy is produced by the phonic lips of the right nasal passage of the spermaceti, the distal and frontal sacs act as sound reflectors, therefore there is a delay in transmitting the reflecting sounds. As pressure builds up behind the phonic lips, these act as check valves so open and close rapidly producing that initial sound energy and projecting the click into the environment.

A sperm whale echolocates its prey

Once the click is released into the environment it travels very fast and over a very long distance. If the sound waves interfere with an object, then they will bounce back. The bounced sound is called an echo. By receiving this echo, the whales have the ability gain information about the size, position and shape of the object. This is termed echolocation. Echolocation is used by whales to locate prey and calculate the distance between them and any obstacle. In an experiment, an Orca whale in captivity, echolocated a 2.5 cm metal object from 75 meters away. Echolocation of whales is getting more difficult every day because of human activity. Sounds produced by boat engines, disposed litter in the open ocean, and even wrecks, all have a different effect. They either alter echo, therefore whales receive false information, or may even delay an echo due to the different consistence of the media that sound travels.

But still not enough are known about the mechanisms of transmitting and receiving these calls, nor their complete role in communication. Scientist are now trying to record and understand repertoires of different pods and species.

References:

1.    Marine Mammals : An evolutionary approach by A. Rus Hoelzel

2.    http://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.2010.00399.x/pdf

7 years ago

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